American football can be brutal. Scientists have long known that the hits, tackles and other collisions that are central to the sport come with health risks, but what these impacts do to the brain has been less clear. What we do know is that many footballers have died with brains that have shown evidence of chronic traumatic encephalopathy (CTE), a disease associated with symptoms such as mood swings and dementia. But because a diagnosis requires that investigators look for damage in a person’s brain after they die, it’s unclear just how common CTE is among current or former athletes.

But now, in a study published in JAMA Network Open, an analysis of medical records from thousands of deceased former NFL players—including hundreds who donated their brains to science—estimates that about 75 percent of these players likely died with CTE.

“No other environmental or occupational exposure ever studied is known to have increased dementia risk at death by that amount,” says Yorghos Tripodis, senior author of the paper and a professor of biostatistics at Boston University.

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The study is based on records of 2,362 former NFL players who died between 2008 and 2023 and played in the league for an average of five years. Among those who donated their brains—almost 400 players—a stunning 93.5 percent had CTE. Signs of CTE in the brain can include atrophy, or tissue wasting, as well as clusters of tau protein—a symptom that is also linked to Alzheimer’s disease.

The high incidence of CTE in those donors may reflect the sample itself: people who donate their brains to science tend to have more severe behavioral symptoms, Tripodis says. That could skew the dataset, potentially making CTE rates appear artificially high.

To better estimate how common CTE might have been across all 2,362 former players in the study, Tripodis and his colleagues analyzed the medical records of donors and nondonors. The team found that people with cognitive symptoms were about five times more likely to donate their brain. Given that, the researchers recalculated their incidence estimate to find that some three in four football players in the study likely had CTE.

For comparison, among the general population, fewer than 1 percent of brains in community-based brain banks show signs of CTE, Tripodis says.

The study only includes NFL players who had died, and it remains difficult, if not impossible, to diagnose CTE in living players. It is hard to say what the risk to NFL players today is, Tripodis says. The NFL has also taken steps to mitigate the risk of head injuries.

The records may also be incomplete or imprecise. For instance, symptoms of CTE such as dementia may not be listed on death records, Tripodis says. And even if dementia is listed, it can have other—even multiple—causes.

This study is “the best attempt so far to correct for selection bias in CTE brain bank data,” says Russell Lonser, a professor in neurological surgery at the Ohio State University, who was not involved in the study. But “it is an estimate from a model and should be read that way.”

“Without similar men who never played contact sports, we can’t tell how much of the pathology or dementia comes from football,” Lonser says. “The 75 percent includes any CTE pathology, even early changes that may never have caused symptoms.”

To better understand how football increases the risks of CTE, Lonser says, you’d need to study players, with and without symptoms, over time, along with conducting brain analyses and including a comparison group of people who had no exposure to head impacts. “Until that work is done, we won’t know how common CTE really is or what, specifically, it means for players.”

Still, Tripodis hopes the findings will prove useful for families who have been affected by CTE and will inform current and future players of the risks involved in the sport.

“CTE is a preventable disease. We find, in study after study, that the more blows to the head over your lifetime, the higher the risk of CTE,” Tripodis says. “If you reduce exposure to repetitive head impacts, then you reduce the risk of CTE.”